The present invention relates to a shaped beam antenna system, in particular, relates to such an antenna system which illuminates the desired shape of area.
A shaped beam is obtained by a paraboloidal reflector with a primary radiator. When a primary radiator is positioned so that the phase center of the same coincides with the focus of the paraboloidal reflector, the wave-front of the reflected wave is along a plane, thus a sharp beam is radiated in the principal axis direction of the parabola. On the contrary, a paraboloidal reflector focuses a plane wave incoming from the principal axis upon the focus. In fact, the radiation pattern of an antenna is the same whether it is used as a transmitting antenna or a receiving antenna according to the reciprocity theorem of an antenna pattern. So, the following description is directed to a transmit antenna, but it should be appreciated of course that a receive antenna is possible in a similar manner.
Conventionally, a reflector has been used so that a wave is radiated sharply in a desired direction, and a minimal wave is radiated in an undesired direction. A pencil beam, or a sharp beam, has been obtained by using a paraboloidal reflector.
On the other hand, when a shaped beam which illuminates a desired shaped service area is requested, a fan-shaped bream is necessary.
One prior art method for providing a fan-shaped beam is the use of a plurality of pencil beams, each of which independently illuminates a different related area.
FIG. 11 shows the conventionally shaped beam antenna system, wherein FIG. 11A is a perspective view; FIG. 11B is a cross section of the array of the primary radiators; and FIG. 11C is the equi-level contour pattern wherein the shape of a beam footprint is shown on a specified surface formed by the intersection of the surface. In FIG. 11A, the reference numeral 101 is a reflector, 102a through 102e are primary radiators, 103a through 103e are feeders for feeding the primary radiators, 104 is a beam forming network, 105 through 105e are element beams, 106a through 106e are equi-level contour pattern of element beams, and 107 is the equi-level contour pattern of the combined shaped beam. The reflector 101 is a paraboloidal reflector, and a 5.times.5(=25) number of primary radiators are used in the embodiment. Each of the element beams 105a through 105e is a pencil beam, and provides the small circle of equi-level pattern as shown by the reference numerals 106a through 106e. When the primary radiators 105a through 105e are simultaneously excited through the beam forming network 104, the whole shaped beam 107 which is the sum of the element beams 106a through 106e is obtained. When the beam forming network 104 adjusts the amplitude and the phase of the exciting signal applied to each primary radiator, a desired shape beam is obtained. The antenna system of FIG. 11 has been used as a satellite antenna for illuminating a desired area on earth.
However, the conventional antenna system of FIG. 11 has a disadvantage in that so many primary radiators (25 radiators in FIG. 11) must be used, and therefore, the same number of feed lines must be used, and the structure of the beam forming network 104 becomes complicated. Further, the minimum spacing between the adjacent element beams is restricted by the physical size of the primary radiators. When the spacing between two adjacent element beams is large, the electric power flux density on earth is not uniform, but the flux density is weak at the gap area between two adjacent element beams. Therefore, it is difficult to provide a shaped beam uniform flux density.
Another prior method for providing a shaped beam is the use of a reflector and signal primary radiator wherein the reflector is a cylindrical paraboloid; i.e., a dense set of parabolas shifted parallel along a predetermined straight line. The just-described prior art method is in accordance with FIGS. 8 through 10.
FIG. 8 shows the reflector of the second prior art method, wherein reference numeral 1 is a reflector; 2 is a reflection plane which is a part of the reflector 1; 2a is an edge of the reflection surface; 3 is a cylindrical paraboloid which composes said reflection surface 2; 4 is a straight line on which vertexes 4a of the parabolas 3a locuses; 4a is a vertex of a parabola 3a. The curved surface 3 is a dense set of parabolas 3a which shift in parallel so that the locus of the vertexes of the parabolas 3a is a straight line 4. In other words, the curved surface 3 is a cylindrical paraboloid in this case. The reflector 1 can provide a fan-beam on an elongated service area. The more advanced technique invented by Dunbar (Calculation of Doubly Curved Reflectors for Shaped Beams Proceedings of the IRE Wave and Electrons Section, pages 1289-1296; October, 1948) for beam shaping consists of forming the reflector 1 from the plane-curve locus 4 which lies on the plane of symmetry. After finding the proper plane-curve locus 4 by Dunbar's method, the reflector 1 producing a flat fan-beam with an arbitrary power pattern over the straight, elongated beam footprint can be obtained.
FIG. 9 shows the shape of the fan-beam of the reflector 1 in the form of the equi-level contour pattern, wherein the horizontal axis shows the horizontal angle and the vertical axis shows the vertical angle. The numeral 8 shows the equi-level contour pattern and the numeral in the figure shows the level in dB.
However, the second prior art method described in accordance with FIGS. 8 through 9 has a disadvantage in that the elongated contour is only in a straight linear shape, and wherein another shape of a fan-beam with a footprint which is a curvilinear contour is impossible. This is explained in accordance with FIG. 10.
In FIG. 10, the reference numeral A(9a) is the position of a reflector; B and C are ends of a target to be illuminated; D is a foot of a perpendicular on the surface which includes the arc BC(9b) from the point A; 7 is a fan-beam having the shape relating to the arc BC, and 9c is a linear line between the points B and C. When a reflector 1 is positioned at the point A(9a), and the object on the arc BC(9b) on the plane with the spacing (h) from the point A is illuminated, the fan-beam must be in the curved shape 7 which is curved similar to the arc (9b). However, the second prior art method which has a reflector 1 with the locus 4 of the vertexes of the parabolas 3a on the plane of symmetry can only provide a linear-shaped beam for the linear line 9c, but not a curved shaped beam for the arc BC. Therefore, the second prior art method cannot provide a curved fan-beam when h is not zero, although it can provide the same when h is zero, even if the locus 4 of the vertexes of the parabolas 3a is not a straight line, but a plane curve on the plane of symmetry.